1. Field of the Invention
The invention relates to SAW (Surface Accoustical Wave) devices generally and more particularly to arrangements for stabilization of such devices.
2. Description of the Prior Art
In the prior art, the so-called SAW devices have found a number of uses. Basically, those devices rely on the generation and detection of surface acoustic waves, also known as RAYLEIGH waves, in a piezoelectric material. Although the piezoelectric phenomenon has been known and used to provide stable signal delay filters and oscillators, the exploitation of the effect had, until comparatively recently, been largely confined to so-called bulk wave applications.
Relatively recently however, the refinement of photo lithographic processing (photoetch techniques) particularly as applied to microcircuits, has made possible the SAW interdigital transducer. Such devices have been described in the technical and patent literature, for example, in U.S. patent application Ser. No. 493,673, filed Aug. 1, 1974, (now U.S. Pat. No. 3,979,700, entitled: "Video Processor". That U.S. patent application describes the application of SAW technology to a particular type of video processing and is assigned to the assignee of the present application.
A more directly pertinent prior art document, in that it relates to SAW oscillators specifically, is the technical paper entitled: "The Surface Acoustic Wave Oscillator -- A Natural and Timely Development of the Quartz Crystal Oscillator", by Meirion Lewis of the Royal Radar Establishment, Malvern, Worcs., UK. That paper was published in the Proceedings of the 28th Annual Symposium on Frequency Control, May 1974, and is distributed by the National Technical Information Service of the U.S. Department of Commerce.
In the aforementioned technical paper, the use of the SAW transducer as the feedback element in an oscillator is shown. The paper also suggests ways of producing frequency control or frequency modulation of the SAW oscillator. The relative difficulty of effecting frequency modulation of such a device can be appreciated in view of the several circuit approaches suggested by Lewis. These approaches include incorporation of a phase shift network in the amplifier circuit of the oscillator loop, use of varactors in a phase shift network, etc.
Piezoelectric devices, and particularly oscillators employing piezoelectric elements, are noted for their relatively high order of stability, and the same benefits accrue to SAW oscillators of the type to which the present invention applies, as generally applied to bulk effect devices. By bulk effect devices, in this case, it is intended to refer to the crystal oscillator of the type well known and widely used in the radio arts for many decades of time.
It will be realized that the introduction of other circuit elements, such as varactors, etc., must inherently deteriorate the stability advantages employed by unmodulated SAW oscillators, because the frequency determination function is partly usurped from the piezoelectric element. Moreover, the additional complication tends to tarnish the attractiveness of the SAW oscillator, which is inherently a very inexpensive and highly satisfactory device capable of being efficiently manufactured in sizes compatible with micro-circuitry.
Still further, U.S. patent application Ser. No. 628,082 filed Nov. 3, 1975, (now U.S. Pat. No. 3,979,697) and titled, "Frequency Modulated SAW Oscillator", also assigned to the assignee of the present application, introduces a technique for altering the frequency of a SAW oscillator by applying a voltage signal end-to-end on the piezoelectric substrate thereby having an effect on the physical length of the said substrate in the direction of propagation of the SAW waves thereon. The frequency control provided in that device is also incorporated into the present device in combination with a unique arrangement for doubly exploiting the characteristics of the piezoelectric substrate to provide greatly improved stability.
In the hereinbefore identified technical paper by Meirion Lewis, an oscillator configuration is described with phase shift compensation substantially only for phase shift in the amplifier which, in combination with the SAW device, operates as an oscillator. The configuration suggested by Lewis requires very accurate placement of the interdigital finger transducers to a precise quarterwavelength offset. The device is moreover, relatively lossy due to the split transducer-PIN diode configuration, and is relatively sensitive to amplifier variations. Those factors tend to further degrade the obtainable performance and stability obtainable in this typical prior art device.
The manner in which the present invention alleviates the problems extant in the prior art by providing a simple, low-cost and efficient SAW circuit, particularly described as an oscillator, but nevertheless adaptable in other SAW circuitry, will be evident as this description proceeds.